The present invention relates generally to sediment control barrier systems and relates more particularly to filtration barriers for waterborne silt and to methods and apparatus for installing the silt filtration barriers.
Sediment control barrier systems have a variety of uses and a wide range of industrial applications. They include: protection of surface soils from surface water erosion; the trapping of sediment; and run-off water filtration for improving water quality and for preventing waterborne silts and solids from entering channeled streams and drainage control systems. A wide variety of materials and structures are used in sediment control barriers systems and in geo-barrier systems in general. Since terminology is somewhat non-standardized, the following terms are defined herein. The term ‘geo-fabric’ refers to a geo-textile, a geo-membrane or a geo-grid structure, or to a combination of thereof. The term ‘geo-textile’ refers to a woven, non-woven, or knitted, biodegradable-resistant fabric that is sufficiently porous as to allow movement of air and water. Geo-textiles are typically load-bearing, synthetic fabrics used as a filter to prevent the passing of fine grained material such as silt or clay. The term ‘geo-grid’ refers to biodegradable-resistant material manufactured into an open, lattice like sheet configuration. Geo-grids are typically made of plastic and used as a reinforcing structure. The term ‘geo-membrane’ refers to essentially impermeable polymeric sheets. Geo-membranes are typically used as hydraulic barriers in liner and cover systems.
Sediment control barriers include silt fences constructed of filtering fabrics, support posts and wire fences. These silt fences typically are single vertical barriers made from a fabric supported in an upright position by posts and support mesh. Silt fences are frequently temporary sediment barriers made of woven synthetic filtration fabric supported by steel or wooden posts. Silt fences prevent sediments carried by un-channeled flow, or sheet flow, of rainwater from leaving a ground site and either entering natural drainage channels or entering waste and storm drain systems. The barriers slow the runoff sheet flow and frequently create a ponding of water upstream of the silt fence. The reduction in water velocity causes the larger entrained soil particles to settle to the ground surface upstream of the silt fence. A silt fence constructed of permeable geo-textile sheets creates a filtration barrier that filters suspended silt particles as the low velocity or ponded water passes through the silt fence to form an effluent stream. The filtered silt particles are shed from the vertical fabric barrier surface or the standard silt fence and accumulate at its base. The size of openings in the barrier determines the size of the particles filtered. The size, shape and number of the openings, as well as the height of the ponded water, determine the flow rate of the filtered effluent stream.
Un-channeled surface water that is deposited upon ground having a sloped surface moves by gravity directed flow along paths determined by the contour of the ground surface. Typically, a silt fence is installed along a path spanning the sloped ground surface or along a path spanning beneath such a slope. The installation path is selected such that it is transverse to the water flow path and impedes the flow. Since sheet flow run-off is stored upstream of a silt fence, the slope grade and the slope length determine the hydraulic load experienced by the silt fence. A maximum recommended slope length upstream of an installed silt fence is determined based on the mechanical strength of the silt fence assembly, the flow rate through the barrier and the volume of water per unit slope area per unit time expected to be deposited upon the slope.
A standard silt fence assembly consists of a woven geo-textile sheet stapled to a series of long wooden stakes. The typical length of the stakes is 48 inches and the stakes are typically made of a hardwood. The stakes are installed at intervals of from 4 to 6 feet and are driven into the ground along a selected path typically over a contour of the surface transverse to the path of ground water run-off. Frequently a portion of geo-textile sheet extends beyond the ground surface as a ground skirt. The ground skirt can either lie on the ground surface in front of the barrier or can be buried in a trench in front of the barrier. The purpose of the ground skirt is to prevent the ponded water from freely flowing beneath the silt fence. Such flows would greatly reduce the effectiveness of the barrier and would tend to erode the ground surface that supports the barrier.
There are a variety of problems associated with standard silt fencing installations. One problem is that standard silt fences cannot effectively filter silt particles that are substantially smaller than the size of the openings in the woven geo-textile sheets. Different types of ground surfaces, when eroded by water runoff, produce different types and sizes of entrained and dissolved sediment particles. Clay soils in particular produce very finely divided silt particles, including suspended particles that are much smaller than the average apparent opening size of between US Standard Sieve size number 20 and US Standard Sieve size number 40, which is a typical range of sizes for standard silt fence fabrics.
Where a standard silt fence is used to filter storm water runoff containing very finely divided silt particles, the silt fence effluent remains turbid, retaining significant portions of the runoffs suspended particles, including colloidal particles such as colloidal clay silt. Besides being aesthetically unpleasant, such turbid effluent can produce significant ecological hazards if allowed to enter surrounding streams, lakes or wetlands, including depletion of dissolved oxygen, reduced photo-penetration depths, and covering of benthic aquatic organisms.
One method known in the art to overcome silt fence filtering limitations is to substantially slow or even pond runoff water to allow waterborne silt particles entrained in the water flow of the runoff to fall out of the water stream and be deposited on the submerged ground surface prior to reaching the silt fence. While effective for larger, entrained particles, this method is much less effective for quasi-colloids, quasi-colloids herein defined as very small entrained particles that require significant amount of time in low velocity conditions to settle out of the runoff water. Ponding and settling is virtually ineffective in removing suspended particles such as colloids and quasi-colloids.
A method that is effective in removing suspended colloids from storm runoff and waste water is to pool the water into basins and seed the water with flocculants. The flocculants react with the colloids to form flocs that settle from the basin waters. However, flocculant treatments require the water to be channeled and, preferably, not flowing. Where the water is not channeled, such as storm runoff deposited upon an un-channeled ground surface, the standard flocculant treatments have only a limited effectiveness. Also, standard flocculant treatments do not efficiently remove suspended particles from channeled but flowing runoff or waste streams without diversion into catchment basins.
A final method of removing suspended particles such as colloidal silts is to force the water runoff through mechanical filters of sufficiently small size as to remove the suspended particles. This method is impractical because of many factors, including the low mechanical strength of such filters, the very long filtration time required, and the high cost of such a system.
What is needed then is a silt fence system that can intercept an un-channeled water flow, efficiently treat the water flow with flocculants, and remove the resulting flocs.
Additionally, what is needed is a silt fence assembly that has a flocculant treatment capacity and is as durable and as simple to install as a standard silt fence.